This invention relates to ophthalmic spectacle lenses for distance vision correction.
Prescriptive ophthalmic lenses used for the correction of hypermetropia are positively-powered lenses, thicker at the center than at the edge. When conventional spherical ophthalmic lenses used for this correction increase in prescription strength, they become thick in the center. Lenses with a smaller diameter can be manufactured to a reduced center thickness. Lenses of this type can be manufactured to larger diameters and edged down to smaller diameters because the edging process does not affect center thickness. However, when the lenses are reduced in diameter by edging, the finished edge thickness increases as the diameter decreases. The increase can be determined by the difference in curvature between the anterior and posterior surfaces.
The appearance and the optical performance of the lens can be improved by providing an aspheric surface of revolution on the lens. This approach can be used to produce lenses to larger diameters. For example, M. Jalie, "The Principles of Ophthalmic Lenses", Chapter 21, 4th Edition, London, The Association of British Dispensing Opticians (1988), describes the optical and cosmetic advantages achieved by lens designs in which one surface is aspheric (typically, but not necessarily, the anterior surface). It is generally desirable that optical performance not be sacrificed when a sphere, or a toroid when correcting for astigmatism, forms the posterior surface. It is also possible to prepare aspheric lenses in a semi-finished form (i.e., as oversized lens blanks), which can be fabricated to a final prescription using optical laboratory machinery.
Aspheric plus-power lenses for correction of hypermetropia that improve optical performance and cosmetic appearance can have spherical or toroidal posterior surfaces. Aspheric surfaces of revolution can be expressed as deformed spheroids, in which the departure from a sphere increases with the distance off-axis. For plus-power lenses (e.g., positive diopter lenses), this departure is in the direction of reduced curvature, below the constant curvature characteristic of spheroids, so that, for example, the center thicknesses of such lenses can be reduced. Examples using deformed spheroids include: lenses having distinct zones of differing curvature that are blended or smoothed in the regions connecting such zones (see, e.g., Bristol, U.S. Pat. No. 5,131,738); lenses having anterior surfaces expressed as surfaces of revolution of polynomials (see, e.g., Davis et al., U.S. Pat. No. 3,960,442); or lenses having anterior surfaces expressed as conicoids (see, e.g., Jalie, U.S. Pat. No. 4,289,387).